“Array-type” sensors, comprising a plurality of individual sensing elements, each element having limited selectivity for a particular chemical species, can be used in selective detection. For example, array-type sensors made from carbon black/polymer composites elements, each element sensitive to many different volatile organic compounds, may selectively detect nerve agent stimulants. As another example, arrays made from resistive metal oxide sensing elements, each element sensitive to many different hydrocarbons, may selectively detect forane.
Array-type sensors may also be useful in emission control of combustion exhausts by sensing and/or characterizing the chemistry of the combustion exhaust. Combustion exhausts are complex mixtures, containing several reducing and oxidizing species, including CO, NOx, and hydrocarbons (CyHz) along with varying amounts of O2, H2O, and CO2. While CO and CyHz can be readily ameliorated by oxidation, NOx is more difficult to remove from exhausts, especially in an O2-containing environment. One technique that is used to reduce the amount of NOx emitted from the exhaust is selective catalytic reduction (SCR), which uses ammonia (NH3) as a reagent. Under appropriate conditions, NH3 reacts with NOx to produce nitrogen (N2) and water (H2O). However, escape (or “slip”) of NH3 from a SCR system may be harmful, so monitoring for NH3 is desirable.
Resistive metal oxide arrays may selectively detect NH3. However, resistive metal oxide sensing elements may be sensitive to varying [O2] and [H2O] because the transduction mechanism in these sensing elements (change in DC electrical resistivity) relies upon changes in the surface concentration of adsorbed species such as O2. Because combustion exhausts often have varying [O2] and [H2O], resistive oxide sensing elements may not be optimal for use with combustion exhausts. Resistive oxide sensing elements may lose their effectiveness above about 500° C.